When a fluid, especially a conductive fluid, such as a liquid metal, is treated, it may be necessary to carry out the heating or reheating of this liquid in a pipeline where the fluid circulates permanently or intermittently.
This purpose is served by using a transformer the primary part of which consists of at least one electrical winding arranged round an arm of a magnetic circuit and the secondary part of which consists of the closed circuit forming part of the pipeline containing the conductive fluid.
This closed circuit forming the short-circuit secondary part of the transformer has passing through it an electrical current generated by induction and circulating either in the wall of the pipeline or in the fluid or both in the pipeline and in the fluid, depending on whether the pipeline or the fluid or both at the same time are electrically conductive. The pipeline and/or the fluid are thus subjected to heating by the Joule effect.
It is necessary to ensure good magnetic coupling, thereby reducing the leakage fluxes, by careful choice of the form and relative location of the primary winding and secondary circuit.
The known prior art provided a heating device fed with single-phase current, consisting of a closed magnetic circuit having a central column, around which is arranged a primary winding fed with single-phase current, and two lateral columns substantially parallel to the central column.
The closed-circuit part of the pipeline consists of two arms connected at each of their ends to the pipeline and forming a loop surrounding the primary winding fed with single-phase current.
The disadvantage of this device of simple construction is that it has limited power for the following reasons:
the hydraulic cross-section of the pipeline must be sufficiently high to ensure that the fluid flow necessary for dissipating the heating power can be established, PA1 on the other hand, this cross-section must be limited in view of requirements associated with the effective electric functioning of the device.
In fact, the magnetic leakage inductance assumes a magnitude which increases, in relation to the electrical resistance of the loop, with the cross-section of the pipeline.
Such a heating or reheating device fed with single-phase current is therefore of limited power, and this can be a considerable disadvantage in some uses.
As regards fast-neutron nuclear reactors using sodium as a heat-exchange fluid, the liquid sodium must be reheated in some parts of the installation, especially in the secondary circuit, in the vicinity of the steam generators. For example, it is necessary for the liquid sodium to be reheated in a pipeline which is branched off from the liquid sodium outlet pipeline from the steam generator, and on which is arranged a hydrogen detector which triggers an alarm when a water leak in one of the tubes of the steam generator results in the presence of hydrogen in the sodium at the outlet of the tubes of the steam generator. In fact, the detection of hydrogen carried out by separation of the hydrogen contained in the sodium makes it necessary to maintain the temperature of the sodium at a particular value ensuring that the detector functions satisfactorily.
It is likewise necessary to maintain the temperature of the liquid sodium near a specific value in some parts of the sodium circuit of the reactor, in order to prevent thermal shocks.
In this case, the fluid is a highly electrically conductive liquid metal and the heating power required can be of the order of approximately ten or a few tens of kW. It is then difficult, if not impossible, to obtain this heating power by feeding the transformer with single-phase current at the industrial frequency of the network.
It has therefore been proposed to use transformers fed with three-phase current, in order to prevent too unbalanced a load on the distribution network.
In this case, the magnetic circuit of the transformer consists of a frame comprising three substantially parallel columns, around each of which is arranged an electrical winding connected to one of the phases of the three-phase current source. The pipeline containing the conductive liquid, such as sodium where fast-neutron nuclear reactors are concerned, has three branched parts, each consisting of one or more turns of a tubular element receiving the liquid to be reheated. Each of the turns of the tubular element is arranged around a transformer winding fed with electrical current from one of the phases of the three-phase current source.
Such a device is of relatively complex construction in terms of the structure of the pipeline, and gives rise to difficulties as regards the heat insulation of this pipeline.